dap: add debug adapter implementation

Adds a simple implementation of the debug adapter that supports the very
basics of a debug adapter.

It supports the launch request, the configuration done request, the
creation of threads, stopping, resuming, and disconnecting from server.

It does not support custom breakpoints, stack traces, or variable
inspection yet. These are planned to be added in the future.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

docs/reference/buildx_dap.md

@@ -0,0 +1,23 @@
+# docker buildx dap
+
+<!---MARKER_GEN_START-->
+Start debug adapter protocol compatible debugger (EXPERIMENTAL)
+
+### Subcommands
+
+| Name                           | Description   |
+|:-------------------------------|:--------------|
+| [`build`](buildx_dap_build.md) | Start a build |
+
+
+### Options
+
+| Name            | Type     | Default | Description                                                |
+|:----------------|:---------|:--------|:-----------------------------------------------------------|
+| `--builder`     | `string` |         | Override the configured builder instance                   |
+| `-D`, `--debug` | `bool`   |         | Enable debug logging                                       |
+| `--on`          | `string` | `error` | When to pause the adapter ([always, error]) (EXPERIMENTAL) |
+
+
+<!---MARKER_GEN_END-->
+